In both of the lightly stocked areas, meadow pipits responded by producing slightly more male chicks than females.

But either heavy grazing or no grazing at all resulted in more females than males.

"We've known for a while that seabirds in particular will change the sex ratio of their offspring depending on environmental conditions, but no-one's looked at this in an applied context," Darren Evans from Hull University, another of the researchers, told BBC News.

"My interest was that given that so many of our birds are in decline, what can we learn that we can bring into a conservation context?"

Double dip

The team suggests that the changes in sex ratio are connected to the quality of the habitat.

If rearing females takes less energy than rearing males, it would be natural that birds in low-quality habitat responded - via mechanisms as yet unclear - by producing more females.

Another idea is that female chicks are the ones that fly far from the nest to look for mates and for their own nesting sites; so rearing more females would lead to a net movement of young birds away from the poor habitat to areas that might be more conducive.

Image caption
Meadow pipits make their nests at ground level, where the chicks' feathers camouflage them

But Dr Evans said it was "not rocket science" to presume that if the population was dominated by one sex, reproduction would suffer.

And other research is also showing there is a need to understand factors affecting reproductive success in more detail, he said.

"Classically what happens is that conservationists will look at how birds are doing in terms of how many chicks they're producing, but what we've found is that birds respond in subtle ways.

"So pipits lay larger eggs in areas grazed by sheep and cattle, and in time we see an increase in breeding sites in those territories - and we think that chicks hatching from larger eggs have a larger chance of fledging and surviving the winter."

It appears that the birds prefer fields that are lightly grazed by a range of animals because this creates a mixture of short and long vegetation.

The long stuff provides secure nesting, while the pipits find it easier to find their insect prey in closer-cropped sward.

The meadow pipit is quite a widespread species, found across the UK, and not limited to farmland; numbers appear healthy.

Birds specialised on farmland, however, are not doing quite as well.

"Basically they're flatlining at the moment," said Gareth Morgan, head of countryside policy with the Royal Society for the Protection of Birds (RSPB).

"From 1970, their numbers declined by about half, and they've been bumping along at that level - recently we've seen signs of them dipping down again, and we don't know why that's happening."

At national and European level, policies have been established aimed at encouraging farmers to look after wildlife on their properties.

As to whether these policies could ever be refined so they encouraged specifics such as the best sex ratio for bird species, Mr Morgan was doubtful.

"That feels like it would be a long way off - I'm more interested in getting the basic schemes right first, " he told BBC News.